1. Field of the Invention
This invention relates generally to wellhead equipment, and in particular to a mechanism for supporting a string of casing in tension within a well.
2. Description of the Prior Art
In an oil and gas well, one or more strings of casing will be cemented within the well. In one system used with offshore jack-up drilling rigs, the string of casing in the well will be supported by a mudline hanger located in a subsea housing at the sea floor. A section of the casing will extend upward to a surface wellhead housing on the drill rig. The surface wellhead housing will be located above the sea and below the rig floor. The distance from the subsea housing to the surface wellhead could be as much as 500 feet with a large jack-up drilling rig.
Cement will be pumped down the string to flow up the annulus to cement the casing in the well. The level of cement will be below the mudline hanger. In the prior art system, the casing will be cut off at the surface wellhead. The blowout preventer will be removed, and a spear will be used to pull tension on the casing after cementing. Then slips will be inserted around the casing which engage the wellhead housing and grip the casing to hold the casing in tension. A packoff will be installed between the casing hanger and the wellhead housing.
A disadvantage of the prior art system is that the blowout preventer must be removed while installing the slips and packoff. A danger of a blowout thus exists. Also, the prior art system is time consuming and expensive. In addition to this, the sealing mechanisms are generally elastomer or on site machined to give metal-to-metal seals.